The power transmitted between two points in an AC electric power transmission system is primarily determined by the magnitude of the voltages at the two points, the angle between the vectors related to these two voltages, and the transfer impedance between the two points. Power is proportional to the voltage magnitudes. However, voltage magnitudes are generally controlled to within relatively small range of specified limits to stay within maximum design limits and to prevent unacceptable voltage variations in the supply voltages provided the utility customer. With large fixed transfer impedances, the degree of control of power by adjustment of voltage is limited. The power transmitted is approximately proportional to the sine of the angle between the two voltage vectors. It is therefore common to control the powerflow by adjusting the angle between the respective voltage vectors. The control of powerflow by controlling this angle between the voltage vectors is generally achieved by the relatively slow adjustment of rotor angles of synchronous machines. In such a case, the maximum permissible angle, and thus the power transmitted, may be limited by considerations of system transient and dynamic stability.
A variety of methods involving shunt reactive devices have been used to increase the power transfer capability and transient and dynamic stability limits. Synchronous condensers, shunt capacitors, shunt reactors, thyristor switched and/or controlled static VAR compensators and saturable reactor compensators are shunt devices used for the purpose. These methods are sometimes referred to as Surge Impedance Compensation or Compensation by Sectioning.
Series capacitor compensation is also sometimes utilized to improve stability limits and increase transfer capability by reducing transfer impedance. This method is sometimes referred to as line length compensation and is essentially a passive compensation technique. Series capacitors have, in a limited way, been switched in and out of the line to enhance stability performance. Otherwise, as a passive device, series capacitors cannot be used for smooth control of transmitted power. Since the transmitted power is inversely proportional to the transfer impedance, the effectiveness of series capacitor compensation to reduce transfer impedance and raise power transfer limits increases with increasing levels of series capacitor compensation. For example, with other factors constant, 50 percent series capacitor compensation reduces transfer impedance to approximately half the original transfer impedance and doubles the maximum power in terms of steady state stability limits. An additional 25 percent compensation that would reduce the transfer impedance to one-fourth the original value would increase the maximum power in terms of its steady state stability limit to four times the original value.
Despite this more than proportional increase in power transfer created by increasing levels of series capacitor compensation, high levels of series capacitor compensation have not heretofore been utilized. It is generally accepted that the practical upper limit of the degree of series compensation is on the order of 80 percent.
High levels of compensation close to 100 percent could produce (1) uncontrollable variations in power or current for small changes in terminal voltages or angles; (2) potentially damaging, undamped, subsynchronous oscillations; and (3) large transient currents and voltages during disturbances due to series resonant conditions.
These large transient currents or voltages may be overcome by the use of gapless metal-oxide varistor (MOV) arresters for overvoltage protection of the series capacitors. Limiting the overvoltage across the series capacitors in such high transient current conditions to the clipping level of the MOV arrester has the effect of changing the effective capacitance value of the series capacitor during such a transient period. This temporarily detunes the series resonance circuit and prevents the transient current from reaching very high values.
The first and second problems mentioned above stem from the problem of lack of adequate controllability of power on the AC transmission system. However, this problem is solved by the present invention.
In interconnected power systems, sometimes there is the problem of unscheduled powerflow through parts of the network due to mismatch between scheduled and actual powerflows. The present invention facilitates adjustment of the relative impedances of different parts of the transmission network to make the actual powerflow closer to the scheduled powerflow.
The present invention further facilitates adjustment of the transfer impedances of various parts of the transmission system to provide powerflow conditions while minimizing losses.